1. Field of the Invention
This invention relates to a grating optical fiber, an optical fiber base material suitable for grating, and method for manufacturing the grating optical fiber.
2. Description of the Related Art
Zero-dispersion fibers (SMF) for a wavelength of 1.3 .mu.m and zero-dispersion fibers (DSF) for a wavelength of 1.55 .mu.m are widely used as transfer optical fibers. If a periodic change (such as a diffraction grating) is provided to the transfer optical fiber, reflection or optical coupling occurs with respect to a specific wavelength. Based on this fact, many studies and researches for forming a grating on an optical fiber in order to extract a specific wavelength have been made.
If the optical fiber is irradiated by ultra-violet rays via a mask having a transparent pattern, the index of refraction of the irradiated portion differs from that of the blocked portion of the optical fiber. By making use of this phenomenon, a grating can be written on the optical fiber. In particular, if the germanium doped in the quartz glass of the optical fiber is irradiated by ultra-violet rays, the lattice defect increases, and therefore, the index of refraction of the portion exposed to the ultra-violet rays increases. Since gratings can be written only in the areas in which germanium is doped, a grating can be formed on the core, which contains doped germanium, when the optical fiber is irradiated by ultra-violet rays.
FIG. 1 is a cross-sectional view of a conventional grating optical fiber taken along the longitudinal axis. This figure also shows the grating writing area 56 and the light propagation range 55. As shown in FIG. 1, light propagates not only through the core, but it slightly spreads out of the core into the clad 54. Even if the optical fiber has a grating on the core which has the filter effect for reflecting a certain light component having a specific wavelength, the filter property is likely to deteriorate because of the leakage of the light into the clad 54.